Preamplifier



QN Mmwkbml Dec- 21, 1954 R. s. JOHN, JR

PREAMPLIF'IER ml N* Nw W EW W hm PREAMPLIFIER Robert S. John, Jr.,Waukegan, Ill., assignor, by mesne assignments, to Panstiehl ChemicalCorp., Waukegan, Ill., a corporation of Illinois Application February17, 1951, Serial No. 211,487

Claims. (Cl. 179100.4)

This invention relates to a preamplifier, especially of the type used inconjunction with phonograph pick-ups. More particularly it relates to apreamplifier for use with a transducer which converts mechanicalVariations into electrical signals.

It is an object of this invention to produce a preamplifier for use witha phonograph pick-up which will give sound reproduction of the highestfidelity. Another object of the invention is to produce a sound pick-uppreamplifier which will give faithful sound reproduction over a widerange of frequencies. to produce a sound reproducing element whereinfeedback control reduces distortion of the sound to a minimum. A furtherobject of the invention is to provide a preamplifying unit in whichfeedback from the plate of a tube to the grid of a tube passes through atransducer and is modified by a circuit having manual impedance control.A further object of this invention is to provide a preamplifier in whicha by-pass capacitance around a resistor in a feedback circuit enableshigh fidelity reproduction of frequencies above 10 kc. A still furtherobject of the invention is to produce a preamplifier in which straycapacitance effects are reduced to a point where they are negligible.

Other features and advantages of this invention will be I apparent fromthe following specification and from the drawing in which:

' Fig. 1 is a schematic diagram of a circuit including the feedbackcontrol of this invention; and Y Fig. 2 is a graph showing the type ofresponse possible under various settings of the manual controls of thedevice of this invention. f

The purpose of a preamplifier used in conjunction with a transducer isto match current recording characteristics of a sound production deviceas closely as possible. It is well known that the ordinary soundreproduction device such as a phonograph is apt to be limited tofrequencies below 10,000 C. P. S. and it is likewise apt to amplifycertain portions of the spectrum to an undue degree while omittingothers entirely, thereby distorting the signal and giving soundreproduction of low quality. In recent years more and more members ofthe public have come to appreciate and insist upon high fidelityreproduction of recorded music. In order to produce sound reproducingapparatus which will give this high fidelity, it is necessary to takeinto consideration the different approaches of the various manufacturersof record discs. Thus, a phonograph which will give very faithfulreproduction of records sold by one manufacturer will not necessarily bein proper balance to give reproduction of equal quality from the recordsof another manufacturer.

Manufacturers have for many years sold records which will produce asignal which is distorted within a certain range of low and highfrequencies. This is done deliberately since it improves the soundquality coming from `the average phonograph. The low frequency levelbelow which signals are distorted is known as the low frequencyturnover. On the high frequency side this point is known as the highfrequency turnover. There are cut-off points on both the high frequencyand low frequency sides. The cut-off points of records sold by differentmanufacturers vary widely. The intentional distortion which is presentin the range between the high frequency turnover and high frequencycut-off on the one hand and the low frequency turnover and low fre-Another object of the invention is quencycut-off on the otherhand mayalso vary in degree. fo

In other words, the slope of the response curve within a distorted rangewill vary among the various manufacturers. In the magazine AudioEngineering of March 1949 at page 14, Messrs. Paul W. St. George andBenjamin B. Drisko list the following information with regard to theturnover points of records put out by several manufacturers of longstanding:

From the above table it may be seen that a phonograph properlycompensated to give good reproduction on a Decca ffrr record would givecomparatively poor reproduction on a Mercury record since their low`frequency turnover points are cycles apart and their high frequencyturnover pointst are 5,000 cycles apart. Applicants preamplifier willgive high fidelity reproduction of any manufacturers production due tothe fact that the preamplifier includes a feedback circuit in which apair of manually controlled potentiometers may be manually adjusted togive the best reproduction irrespective of the manufacturing source ofthe record being played.

While applicants preamplifier is particularly useful in conjunction witha strain sensitive resistance type pick-up it will work withsatisfactory results with other types of sound reproducing transducers.Referring to Fig. 1 a pick-up device 10 is connected to the grid 11 of avacuum tube 12 through a condenser 13. Plate current for the tube 12 issupplied from a 300 volt source 14 shown at the right side of thediagram. The tube 12 may be a pentode of the type 6AU6, 65H7, 6SJ7 or atube having characteristics similar to any of these. A feedback circuitconnects a plate 15 of the tube 12 through a resistor 16 and by-passcondenser 17 back to the pick-up or transducer 10.

A variable impedance circuit indicated generally at 18 is `shown to theleft of the transducer 10. A capacitor 19 and variable resistor20 areconnected in series across the transducer 10. Also connected across thetransducer 10 are a fixed resistor 21 and a variable resistor 22 as wellas a capacitor 23 in series. The capacitor 19 and variable resistor 20are connected in parallel with the resistors 21 and 22 and the capacitor23. The variable impedance circuit 13 serves to modify feedback signalspassingbetween the plate 15 and the grid 11 of the tube 12. Since thefeedback circuit is inverse, the circuit is arranged so as to provideless feedback at the higher frequencies thereby giving a higher responseat these frequencies for the stage as a whole.

Thus, the capacitor 19 attenuates the feedback signal more at highfrequencies in order to provide a treble boost. The variable resistor orpotentiometer 20 serves to vary continuously the upper turnoverfrequency by limiting the action of capacitor 19 at some high frequencywhere the effect of the capacitor 19 becomes negligible compared to thepotentiometer 20. With the potentiometer 20 at maximum the lowest trebleturnover point is about 1500 C. P. S. where the value of the capacitor19 is 0.008 af. andthe value of the resistor 20 is 25,000 ohms. Thepotentiometer or resistor 22 serves to vary continuously the lowerturnover frequency by changing the impedance of the member 1S.Decreasing the potentiometer 22 raises the bass turnover. The resistor21 serves merely to limit the uppermost bass turnover. If the resistor21 is of about 39,000 ohms the uppermost bass turnover will be about 800C. P. S. With the potentiometer 22 at maximum, the lowest turnoveravailable is about 300 C. P. S. for the value of the capacitor 19previously given. Thus, a continuous range of turnover control .isavailable from 300-800 C. P. S. in the bass and from 1,50010,000 C. P.S. in the treble. A-capacitance 23 serves merely to prevent any directcurrent frompassing through the resistors 21 and 22.

The pick-up or transducer requires energization .by a voltage across itof about 50 volts D. C. This polarizlng voltage is applied from theplate of the tube 12 rather than the plate supply 14 and the pick-upitself then serves also as the path by which the feedback signal isreturned to the grid 11. The isolating resistance normally required inseries with the grid circuit in plate-to-plate feedback circuits is thuseliminated. There is in consequence no attenuation of the input signalby such an isolating resistance. At the same time the requiredpolarizing voltage is obtained from the source of the feedback voltagerather than a voltage dropping resistor from the plate .su l

L'algh feedback resistor 16 is by-passed by the capacitance 17. Theresistor 16 is of about 220,000 ohms resistance and the capacitor 17 isabout 10-15 auf. capacitance. Since there is no grid isolating resistorin the grid circuit the stray capacitance effects are reduced to aminimum by the circuit shown. With this circuit there is less feedbackat extremely high frequency, a factor which tends to hold up theresponse at this level.

The remaining circuit shown in Fig. l is standard for a preamplifiercircuit and will not be discussed in great detail. The output from theplate 15 is passed: to the grid of a triode 24. The output from theplate of the triode 24 is passed through a variable resistor to the gridof a triode 25. Both the triodes 24 and 25 may be of the type known tothe trade as 12AU7 or 6SN7. Other tubes having comparablecharacteristics may be substituted. The output of the triode 25 may godirectly .v

to a standard amplifier (not shown).

The values to be assigned to the various resistors and capacitors notdiscussed are well known to those skilled in the art; however, in orderto dispel any doubt as to the suitable values of these various elementsthey are i given numerals running from to 47 and the values which haveproved satisfactory are listed below opposite the numeral assigned inFig. l.

.30 meg-- 1.2

31 ohms 1200 35 meg 1.2

36 ohms 1500 Fig. 2 shows a response curve of db vs. frequency .plottedon semi-logarithmic paper where the frequency component is plotted onthe logarithmic scale. As shown, decreasing the potentiometer 22increases the response in the base frequency range thereby raising thebass turnover whereas increasing the potentiometer 20 decreases thetreble turnover available.

The variable impedance element 18 is shown as being preferably connectedacross a transducer or pick-up device 10. It is obvious that it could beconnected in series with this element in the feedback circuit. Thetransducer 10 may be a strain-sensitive element of the character shownin Germeshausen Patent No. 2,373,676 and John Patent No. 2,492,429, forwhich type of pick-up this circuit is particularly advantageous; or itmay be a magnetic pick-up or other type of pick-up wherein an impedanceis varied to produce the signal. lt is obvious that my invention iscapable of many modifications and it is my intention therefor to belimited only by the scope of the invention as disclosed in the appendedclaims.

I claim:

l. A preamplifier of the character described comprising: a transducerfor converting mechanical variations in a recording member intoelectrical signals, said transducer including a variable impedance; avacuum tube having a plate and grid; a circuit connecting said grid tosaid transducer; a feedback circuit from the plate to the grid of saidtube completed through the impedance 4 circuit of said transducer; and acontrol circuit having an adjustable impedance therein connected to thefeedback circuit to regulate the amount of feedback at certainpredetermined frequencies.

2. A preamplifier of the character described, comprising: a variableimpedance transducer for converting mechanical variations intoelectrical signals; a vacuum tube having a plate and grid; a circuitconnecting the grid of said tube to said transducer; a feedback circuitfrom the plate to the grid of said tube completed through the impedancecircuit of said transducer; and a variable impedance element connectedacross said transducer, sa1d Variable impedance element serving tomodify the amplitude of signals in certain frequency ranges passlngthrough said feedback circuit.

3. A preamplier of the character described, comprising: a transducer forconverting mechanical variations into electrical signals, saidtransducer including a variable impedance; a vacuum tube having a plateand grid; a circuit connecting the grid of said tube to said transducer;a feedback circuit from the plate to the grid of said tube completedthrough the impedance circuit of said transducer; and an adjustableimpedance element connected in series in said feedback circuit toregulate the amount of feedback in certain regulating frequencies.

4. A preamplifier of the character described, comprising: a strainsensitive transducer for contacting the grooves of a record andconverting mechanical variations therein into electrical signals; avacuum tube having a plate and grid; a circuit connecting said grid tosaid transducer; a feedback circuit from the plate to the grid of saidtube completed through the strain sensitive element of said transducer;and an adjustable impedance element connected across said strainsensitive element to regulate the amount of feedback passingtherethrough at certain predetermined frequencies.

5. A preamplifier of the character described, comprising: astrain-sensitive transducer for contacting the waves of a record andconverting mechanical variations therein into electrical signals; avacuum tube having a plate and grid; a circuit connecting said grid tosaid transducer; a feedback circuit from the plate to the grid of saidtube completed through the strain sensitive element of said transducer;and an adjustable impedance element connected in series in said feedbackcircuit to regulate the amount of feedback at certain predeterminedfrcquencles.

6. A preamplifier of the character described comprising: a transducerfor converting mechanical variations into electrical signals; a vacuumtube having a plate and grid; a circuit connecting the grid of said tubeto said transducer; a feedback circuit from the plate to the grid ofsaid tube completed through said transducer; and a variable impedancecircuit connected across said transducer, said variable impedancecircuit having a capacitance in series with a first variable resistorand a second variable resistor connected across said capacitance andfirst variable resistor, said feedback circuit cooperating with saidvariable impedance circuit whereby feedback signals in the treble rangeare attenuated in accordance with the setting of said first variableresistor and feedback signals in the bass range are varied by saidsecond variable resistor, said feedback signals being passed throughsaid transducer to the grid of said vacuum tube.

7. A preamplifier of the character described comprising: a transducerfor converting mechanical variations into electrical signals; saidtransducer including a variable impedance; a vacuum tube having a plateand grid; a circuit connecting said grid to said transducer; a feedbackcircuit from the plate to the grid of said tube connected through theimpedance circuit of said transducer; a resistor in said feedbackcircuit; a capacitance shunting said resistor; and a control circuithaving an impedance which varies as a function of the signal passingtherethrough, said control circuit being connected to the feedbackcircuit to regulate the amount of feedback at said predeterminedfrequencies.

8. A preamplifier of the character described compris-r ing: astrain-sensitive transducer for converting mechanical variations intoelectrical signals, said transducer including a variable impedance; avacuum tube having a plate and grid; a circuit connecting said grid tosaid transducer; a feedback circuit from the plate to the grid of saidtube completed through the impedance circuit of said transducer; aresistor in said feedback circuit; a

capacitance by-passing said resistor; and means connected to saidfeedback circuit for modifying a feedback signal passing therethrough inaccordance with the frequency thereof.

9. A preamplifier of the character described, comprising: astrain-sensitive transducer for converting mechanical variations into anelectrical signal, said transducer including a variable impedance; avacuum tube having a plate and grid; a circuit connecting said grid tosaid transducer; a variable RC circuit connected across said transducer,said RC circuit having a capacitance in series with a first variableresistor and a second variable resistor connected across saidcapacitance and first variable resistor; and an inverse feedback circuitconnecting the plate of said tube to said grid through said transducer,said feedback circuit cooperating with said RC circuit whereby feedbacksignals in the treble range are attenuated in accordance with thesetting of said first variable resistor and feedback signals in the bassrange are varied by the impedance control of said second variableresistor.

10. A preamplifier of the character described, comprising: astrain-sensitive transducer for converting the rnechanical variations ofa record to electrical signals; a vacuum tube having a plate and grid; acircuit connecting said grid to said transducer; a variable RC circuitin parallel with said transducer; a first capacitance in said RCcircuit; a iirst potentiometer in said RC circuit in series with saidrst capacitance; a second potentiometer; a second capacitance in serieswith said second potentiometer, said first capacitance and potentiometerbeing connected in parallel with said second capacitance and p0-tentiometer; and an inverse feedback circuit connecting the plate to thegrid of said tube through said transducer, said feedback circuitcooperating with said RC circuit whereby feedback signal impulses in thetreble range are attenuated in accordance with the setting of said rstpotentiometer and feedback signals in the bass range are varied inaccordance with the setting of said second potentiometer.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 2,328,478 Mason Aug. 31, 1943 2,372,956 Jordan Apr. 3, 19452,373,676 Germeshausen Apr. 17, 1945 2,568,797 Eland Sept. 25, 1951

